The of retinoblastoma susceptibility gene product (pRb) is a potent tumor supressor that is frequently mutated in numerous malignancies including leukemias. pRb and related proteins, p107 and p130 binds to and inactivates different members of the E2F family of transcription factors. In addition to E2F pRb can interact with other factors that modulate transcriptional activity of the pRb-E2F complexes such as: HBP1 (HMG-box- containing transcriptional repressor), hBRGl and hBRM (mammalian homologues of SNF2/SWI2 yeast transcription activator), and histone deacetylase (the enzyme involved in chromatin structure rearrangement) have all been reported to interact with pRb and to modulate its transcriptional activity. Binding of each of these modulators to pRb occur in an LXCXE motif-dependent manner. The LXCXE motif was originally found in certain viral oncoproteins (SV40 T antigen, HPV-16 E7, adenoviral E1A) that also are known to bind to pRb. Preliminary results from our laboratory raised a possibility that pRb may also interact with the HMG1/2 class of proteins. The HMG proteins are nuclear, non-histone structural elements of chromatin that contain an LXCXE motif. We propose to investigate whether the HMG class of proteins contributes to the pRb-dependent regulation of transcription. To address this question we will study specific association of HMG2 and pRB in vitro and in vivo model systems. To investigate the functional significance of HMG2/pRb interaction we will examine effect of HMG on pRb ability to repress transcription in model assays.